1. Field of the Invention
This invention relates to the field of electromagnets. More specifically, the invention comprises a resistive magnet with radial ports providing access to the central region.
2. Description of the Related Art
The present invention proposes to create an electromagnet having a split at the mid-plane in order to allow clear radial access to the magnet's core. Several approaches may be useful for constructing such a magnet. It is therefore important for the reader to understand some known techniques for electromagnet construction prior to receiving the description of the present invention.
A good discussion of prior art construction techniques for high-field resistive magnets is found in an article written by one of the present inventors: Mark D. Bird, “Resistive Magnet Technology for Hybrid Inserts,” Superconductor Science and Technology, vol. 17, 2004, pp. R19-R33. Discussions of prior art construction techniques for high-field split resistive magnets are found in two articles published in the IEEE Transactions on Magnetics: Robert J. Weggel and M. J. Leupold, “A 17.5-Tesla Magnet with Multiple Radial Access Ports,” vol. 24, no. 2, March 1988, pp. 1390-1392; and Pierre Rub and G. Maret. “A New 18-T Resistive magnet with Radial Bores,” vol. 30, no. 4, July 1994, pp. 2158-2161.
The basic principle of an electromagnet is that a conductor must be wrapped around a central bore for one or more turns. Many turns are typically used. FIG. 1 shows an electromagnet created by wrapping conductor 100 around central bore 104 in a helical path. The two ends of the helical path may be provided with a flat 30 to facilitate mounting the coil. Helical gap 28 is typically filled with an insulator of some sort to ensure that the current flows through the helical path.
The version shown in FIG. 1 does not show any cooling channels. For the conductor to carry large currents, cooling channels would need to be added. Radial cooling channels can be cut using wire EDM to make a “Monohelix” magnet as described by Weggel in “The Monohelix: 1) Five Years of Operation at the Francis Bitter National Magnet Laboratory” and 2) Finite Element Stress Analysis,” IEEE Trans. on Magn., vol. 28, no. 1, January 1992. In the alternative, axial cooling channels can be cut via micro-hole and wire EDM to make a Florida-Helix as described by one of the present inventors, Mark D. Bird, in “Florida-Helix Resistive Magnets,” IEEE Trans. on Applied Supercond., vol. 14, no. 2, 2004, pps. 1271-1275. As a Florida-helix is a recent development, the drawing figures depicting it are not described as “prior art.”
The electrical current passing through the helix during operation generates Lorentz forces and considerable heat. Other components are needed to accommodate these factors. The whole device is placed within a surrounding jacket, so that a pressurized fluid can be pumped through the cooling channels. Mechanical attachment features are generally also provided. For purposes of visual clarity, these features have been omitted in FIG. 1.
Bitter-disk type electromagnets are another known approach to carrying high currents. While it is true that those skilled in the art are familiar with the design and construction of such magnets, a brief explanation of the prior art may be helpful. FIG. 2 shows a prior art Bitter-disk magnet. End plate 40 is the anchoring point for a number of circumferentially-spaced tie rods 44. In practice tie rods 44 have uniform length. Some of these are shown cut away in order to aid visualization of other components. A Bitter-disk magnet is typically constructed by stacking the components. Starting with end plate 40, tie rods 44 are added. A series of conducting disks 36 are then slipped onto tie rods 44. The reader will observe that each conducting disk 36 has a series of holes designed to accommodate tie rods 44. Conducting disks 36 are made of thin conductive material, such as copper or aluminum.
Turning briefly to FIG. 4, the reader may observe conducting disk 36 in more detail. Tie rod holes 46 are uniformly spaced around its perimeter. Cooling holes 54 are also uniformly spaced about conducting disk 36. Cut 52 is a radial cut extending completely through one side of the disk. The reader will observe that the two sides of the disk have been displaced vertically, with the result that conducting disk 36 forms one turn of a helix having a shallow pitch. Upper side 50 of cut 52 is higher than lower side 48. The importance of this fact will become apparent as the construction of the device is explained further.
Prior art Bitter magnets are made in several different ways. The specifics of the prior art construction techniques are not critical to the present invention, since the present invention could employ Bitter stacks constructed using any of the prior art techniques. However, in order to aid the understanding of those not skilled in the art, one of the prior art construction techniques will be discussed in detail.
Returning now to FIG. 2, the reader will observe that six conducting disks 36 are initially placed over tie rods 44 (the lowest part of the stack in the view). For the specific version shown, as each conductive disk is stacked, it is indexed 1/15 turn in the clockwise direction (corresponding to the fact that there are 15 tie rods 44). Turning to FIG. 5, the effect of the rotational indexing may be more readily observed.
Six conducting disks 36 have been assembled to create one conductor turn 42. Conducting disks 36 have also been “nested” together. The 1/15 turn is a somewhat arbitrary figure. They could be indexed in other increments. Rotational indexing as large as ⅓ turn is in common use, especially for smaller diameter stacks. In fact, it is more customary to divide the 360 degrees found in one complete turn into even increments. If six stacked conductors are used to make one turn, then it would be common to rotationally index each disk ⅙ turn over its predecessor (60 degree index per disk).
The disks are nested in the manner shown, so that upper side 50 of one conductor disk 36 lies over upper side 50 of the conductor disk 36 just below it. The disks in FIG. 2 are shown with a significant gap between them. The Bitter-disk assembly method squeezes the disks tightly together when the device is complete. The squeezing is typically accomplished by threading the ends of the tie rods. Rigid end plates are slipped over the tie rods at the top and bottom of the stack. Nuts are then threaded onto the exposed ends of the tie rods and tightened to squeeze the end plates toward each other. When squeezed together, conducting disks 36 form one integral conductor having a helical path—albeit with a very shallow pitch.
Still looking at FIG. 2, the description of the prior art device will be continued. The reader will observe that four conductor turns 42 are shown in the assembly (in the uncompressed state). In reality, many such conductor turns 42 will be stacked onto tie rods 44. The desired result is to accommodate a large electrical current flowing through a helix having a shallow pitch. The desired path of current flow commences with one end plate 40 (which makes contact with the underside of the lowermost conducting disk 36). A second end plate 40 (not shown) will form the upper boundary of the assembly (“sandwiching” the other components in between). The current will then exit the device through the upper end plate 40 (The tie rods are electrically isolated from the end plates and the disks so that they will carry no current). Those skilled in the art will realize that if one simply stacks a number of conductor turns 42 on the device, the electrical current will not flow in the desired helix. Rather, it will simply flow directly from the lower end plate 40 to the upper end plate 40 in a linear fashion. An additional element is required to prevent this.
Insulating disks 34 are placed within each conductor turn 42 to prevent the aforementioned linear current flow. Each insulating disk 34 is made of a material having a very high electrical resistance. The dimensional features of each insulating disk 34 (tie rod holes, cooling holes, etc.) are similar to the dimensional features of conducting disks 36. Each conductor turn 42 incorporates at least one insulating disk 34 nested into the stack. FIG. 3 shows a detail of this arrangement. The reader will observe the upper portion and lower portion of each insulating disk 34 (both ends of each disk are labeled as “34” in the view so that the reader may easily distinguish them from conducting disks 36). The reader will also observe how each insulating disk 34 nests into the helix formed by the six conducting disks 36.
FIG. 5 also illustrates this arrangement. Insulating disk 20 is placed immediately over the first conducting disk 36. It then follows the same helical pattern as the conducting disk 36. Returning now to FIG. 2, the cumulative effect of this construction will be explained. The four conductor turns 42 shown in FIG. 2 are identical. When they are compressed together, the four insulating disks 34 will force the current to flow through one continuous helix through the stacked conducting disks 36. Thus, the construction disclosed forces a helical flow of electrical current through the device. An actual Bitter magnet might include 20 or more such conductor turns.
Those skilled in the art will realize that when a substantial electrical current is passed through Bitter magnet 32, strong mechanical forces are created (Lorentz forces). Significant heat is also introduced through resistive losses. Thus, the device must be able to withstand large internal mechanical forces, and it must also be able to dissipate heat. Once the entire device is assembled with the two end plates 40 in place, the end plates are mechanically forced toward each other. The lower ends of tie rods 44 are attached to the lower end plate 40. The upper ends typically pass through holes in the upper end plate 40. The exposed upper ends are threaded so that a set of nuts can be threaded onto the exposed ends of tie rods 44 and tightened to draw the entire assembly tightly together. In this fashion, the device is capable of resisting the Lorentz forces, which tend to move the disks and other components relative to each other. Not all Bitter-type magnets use tie rods. Other mechanical structures can be used to align the components and resist the Lorentz forces. However, since tie rods are the most common approach, they have been illustrated.
Because Bitter magnet 32 generates substantial heat during operation, natural convective cooling is generally inadequate. Forced convective cooling, using deionized water, oil, or liquid nitrogen is therefore employed. A sealed cooling jacket is created by providing an inner cylindrical wall bounded on its lower end by the lower end plate 40, and bounded on its upper end by the upper end plate 42. An outer cylindrical wall is provided outside the outer perimeter of the disks, extending from the lower end plate 42 to the upper end plate 42. All the components illustrated are thereby encased in a sealed chamber. The cooling liquid is then forced into the cooling jacket, where it flows from one end of the device to the other through the aligned cooling holes in the stacked disks (the cooling holes align in the conducting and insulating disks). In FIG. 2, the cooling flow would typically be linear from top to bottom or bottom to top.
Those skilled in the art will realize that the completed Bitter magnet 32 will generate an intense magnetic field within the cylindrical cavity within the inner cylindrical wall. Those skilled in the art will also realize that it is possible to generate an even greater magnetic field by nesting concentric Bitter-type coils. All these components are well known within the prior art of high-field resistive magnet construction. However, the reader should be aware that the history of high-field split magnet construction is much more limited, with only a few magnets having been built. Most of these were built by Weggel and Rub. In both those cases, radial access ports are provided by “interrupting” the Bitter coil at the mid-plane and introducing a copper or brass “mid-plate” that includes the access ports along with the water channels.
The conducting disk shown in FIG. 4 uses round tie rod holes and round cooling holes. Any discontinuity in the cross section of the disk causes structural weakness and imperfections in the magnetic field produced. Viewed only from the standpoint of electromagnetic efficiency, the disk would ideally have no holes at all. Such a design would be impractical, however, since it could not be effectively cooled. The lack of tie rods would also prevent the disks being effectively aligned and clamped together in order to resist Lorentz forces. Thus, the design of a Bitter-type magnet inherently involves compromises between purity of the magnetic field, conductivity, mechanical strength, cooling, and other factors.
In recent years the traditional Bitter disk design has been improved to remedy some of its shortcomings. FIG. 6 shows a conducting disk developed at the National High Magnetic Field Laboratory in Tallahassee, Fla., U.S.A. This type of disk is now known as a Florida-Bitter disk.
As the tie rods are loaded primarily in tension, a non-round shape can be used. An elongated cross section for the tie rod provides a better compromise between the strength required and the space consumed. Such tie rods are now used. Florida-Bitter disk 56 has elongated tie rod holes 58 to accommodate the modified cross section of the tie rods. The shape of the tie rods conform to the shape of the holes illustrated.
Elongated cooling holes also provide a more advantageous strength versus cooling compromise. Florida-Bitter disk 56 has cooling slots 60 in place of the conventional cooling holes. A series of such cooling slots are placed in concentric rings across the width of the disk.
FIG. 7 shows a detailed view of a portion of Florida-Bitter disk 56, wherein these features can be seen more clearly. The reader will observe that successive circumferential arrays of cooling slots are staggered. If one starts with the innermost array of slots, the next outward array is staggered so that the slots in that array are outboard of the webs (the solid material between the slots) in the preceding array. This staggering of the cooling channels substantially enhances the mechanical strength of the conductor allowing higher fields to be attained. It is an important feature of the Florida-Bitter disk.
From these descriptions, the reader will gain some understanding of the construction of high-field resistive magnets. All these techniques can potentially be used in constructing a magnet according to the present invention, which contemplates providing radial access ports which are approximately perpendicular to the central axis running through the magnet's core.
The inclusion of a radial access port is known within the art. Technical articles have described such designs, including: R. J. Weggel, M. J. Leupold, “A 17-Tesla Magnet with Multiple Radial Access Ports”, IEEE Transactions on Magnetics, Vol. 24, No. 2, March 1988; and P. Rub, G. Maret, “A New 18 T Resistive Magnet with Radial Bores”, High Magnetic Field laboratory, Grenoble, France.
Prior art radial port designs have focused on Bitter stacks using radial cooling, meaning that the cooling flows from the central bore out to the magnet's perimeter (rather than longitudinal cooling in a direction parallel to the magnet's central axis). Some type of spacer plate is typically added in the magnet's mid-plane. FIG. 20 shows two spacer plates 106. Each has a pair of radial access grooves 108. Each also has an array of radial cooling channels 110. When the two spacer plates shown are forced together and sealed within suitable cooling fluid manifolds, the two radial access grooves provide access to the magnet's core in the transverse direction.
FIG. 20 shows a very simplified incarnation of this concept. The spacer plates are clamped in the middle of stacks of Bitter disks. The Bitter disks typically have etched radial cooling passages. Those skilled in the art will readily appreciate how the use of radial cooling passages facilitates the inclusion of radial access ports, since the cooling passages and the access ports both proceed from the magnet's core to its perimeter. The present invention seeks to add radial access ports to a coil using longitudinal cooling. A different approach is therefore needed.
Those skilled in the art will realize that the spacer plate shown in FIG. 20 does not allow helical flow of the electrical current. It is simply an electrical “shunt” which passes the current from the Bitter stack clamped on the top of the spacers to the Bitter stack clamped on the bottom of he spacers. This current—which would flow from top to bottom in the orientation shown in the view—does not contribute to creating a magnetic field in central bore 104. Thus, the design shown in FIG. 20 sacrifices some field strength. The reader may naturally wish to know the significance of this sacrifice, since the relatively brief interruption in the helical current path caused by the inclusion of the spacer plates may not be intuitively significant.
FIG. 21 is a cross section through ¼ of a Bitter-type resistive magnet. Only the upper right quadrant is shown. The cross section is of course symmetric about the X axis (labeled as “RADIUS” in the view) and the Z axis (which correspond to the central axis of the magnet). Thus, the X axis shown in the view is the magnet' mid plane. If spacers such as shown in FIG. 20 are used, they will be placed on the X axis in the view.
The figure depicts the winding of a 30 Tesla magnet, using three concentric Bitter coils (first Bitter coil 80, second Bitter coil 82, and third Bitter coil 84). The Bitter coils are divided into regions. The contribution of each region—stated in Teslas per Megawatt—is then shown for each region. As an example, the region of first Bitter coil 80 actually lying next to the magnet's mid-plane, contributes 7.24 T/MW. From even a cursory inspection of this figure, one can conclude that the contribution of conductive turns lying near the magnet's mid-plane to the overall magnetic field produced is substantial. Thus, any sacrifice of turns in this area has a large impact. This fact represents a crucial disadvantage of the approach shown n FIG. 20. Thus, a new construction which can provide access ports through the mid plane while retaining the helical current path would be desirable.